The invention relates to electronic test equipment. In particular, the present invention relates to calibration of electronic test equipment systems such as vector network analyzers.
A network analyzer is a test system that characterizes the performance of radio frequency (RF) and microwave/millimeter wave DUTs in terms of network scattering parameters. Network scattering parameters, more commonly known as xe2x80x98S-parametersxe2x80x99, are transmission and reflection (T/R) coefficients for the DUT computed from measurements of voltage waves traveling toward and away from a port or ports of the DUT. In general, S-parameters are expressed either in terms of a magnitude and phase or in an equivalent form as a complex number having a real part and an imaginary part. For example, a set of four S-parameters, namely S11, S12, S21, and S22 each represented by a complex number, provide a complete characterization of a linear RF performance of a given two-port DUT at a single frequency. A network analyzer capable of measuring both the magnitude and phase of the S-parameters of the DUT is called a vector network analyzer (VNA).
As with all test equipment, VNAs can and do introduce errors into measured S-parameter data. The presence of these errors distorts or corrupts the measurement of actual S-parameters of the DUT by the test system. The characterization and subsequent removal of the effects of systematic errors is often called error correction or calibration. Generally, a VNA calibration involves determining values for error coefficients associated with an error model of a measurement system. For calibration purposes, the xe2x80x98measurement systemxe2x80x99 generally includes the VNA along with any cables, adapters, test fixtures that are to be employed while testing a DUT. Thus a VNA error model attempts to account for all, or at least the most significant, sources of the systematic errors in terms of constituent error coefficients of the error model. Once determined through VNA calibration, the error coefficients are used in conjunction with the error model to mathematically correct for the effects of the systematic errors in the measured S-parameter data for the DUT produced by the VNA. The data after calibration-related correction is typically called xe2x80x98calibrated dataxe2x80x99 and represents a more accurate indication of actual performance of the DUT than uncalibrated or raw data.
All of the major systematic errors associated with using a VNA to measure S-parameters can be accounted for by six types of errors: directivity and crosstalk related to signal leakage, source and load impedance mismatches related to reflections, and frequency response errors related to reflection and transmission tracking within test receivers of the network analyzer. Thus, for a VNA measuring S-parameters of a general two-port DUT, there are six forward-error terms and six reverse-error terms for a total of twelve error coefficients or terms (including two terms that combine the various transmission crosstalk terms into a forward crosstalk or a reverse crosstalk term). Such a full measurement calibration for a general two-port DUT is often referred to as a xe2x80x98twelve-termxe2x80x99 error correction or calibration using a twelve-term error model. An extension of the twelve-term error model for a full measurement calibration of a multiport network analyzer (i.e., a network analyzer having more than two ports) often is referred to as a twelve-term error model also, even though such an error model necessarily has more than twelve terms.
In addition to the twelve-term error models, simpler and in some cases, more accurate error models known as xe2x80x98eight-termxe2x80x99 error models have been developed and are routinely used in situations and under constraining circumstance that allow their use. The eight-term error models actually include two additional terms, for a total of ten, when crosstalk is considered. Thus, the eight-term error model includes two fewer error terms than the twelve-term model when considering a two-port network analyzer or a two-port DUT. A principal difference between the eight-term and twelve-term models is that the twelve-term model has a separate error term for a source match and a load match at each test port of the VNA. Eight-term models, and extensions of such models to multiport VNAs, have only a single match term for each test port of the VNA.
Unfortunately, due to the presence of a single port match error term, eight-error terms are unable to explicitly account for actual differences in the source match and the load match at a test port of the VNA for many VNA configurations. In particular, a VNA may be completely and correctly calibrated using an eight-term error model only if the equivalent test port source match and load match are equal. As such, there are often severe limitations to the applicability of eight-term error models for VNA calibration.
Since VNA calibration using an eight-term model often offers significant practical advantages relative to calibration using a twelve-term model, approaches have been developed to circumvent or overcome the inadequacy of the eight-term model to account for source/load differences. In some cases, an eight-term calibration may be employed when the source/load differences are small enough to simply ignore. In other cases, the difference is accounted for by a mathematical compensation using additional measurements of the test system. Conventionally, accounting and compensating for source/load match differences through the use of additional measurements require supplementary test hardware, such as the use of precision, broadband dual reflectometers at each test port. Thus, the use of an eight-term error calibration for a VNA with a source/load match difference at one or more test ports generally requires a more expensive test system.
Accordingly, it would be advantageous to compensate a calibration of a VNA for the effects of differences in test port source and load match, especially when employing a calibration that does not inherently account for such differences. Moreover, it is desirable that such a compensated calibration not require the use of dual reflectometers at each of the test ports. Such a compensated calibration would solve a long-standing need in the area of VNA calibration and, in particular, in the area of VNA calibration using eight-term error models.
The present invention compensates a calibration of a vector network analyzer (VNA) using an error model calibration. In particular, the present invention compensates or corrects for differences in a source match and a load match of a test port of the VNA, where the differences are otherwise unaccounted for by the error model being employed. In addition, the present invention also may compensate for differences in a directivity transmission tracking term and a reflection tracking term of the error model associated with a test port of the VNA due to switching of receivers at the test port. The present invention may be used to compensate any eight-term model-based VNA calibration or multi-port extension thereof such that accurate, xe2x80x98calibratedxe2x80x99 measurements of a device under test (DUT) are produced by the VNA employing the calibration.
In an aspect of the present invention, a method of compensating a calibration of a VNA is provided. The method comprises characterizing a source match and a load match of a test port of the VNA. The method of compensating further comprises computing a test port delta-match factor from the characterized source match and load match. The method applies to a multiport VNA having two or more test ports.
In some embodiments, the method of compensating a calibration further comprises correcting or compensating measured S-parameters data for a device under test (DUT) using the delta-match factor. Raw or uncalibrated S-parameter data for the DUT is first compensated for the source/load match difference using the delta-match factor to generate compensated raw S-parameter data. Then the compensated raw S-parameter data is corrected for systematic errors through the application of a multi-term calibration correction to produce both compensated and calibrated S-parameter data. In other embodiments, the method of compensating a calibration further comprises modifying or compensating error coefficients of the multi-term error model using the delta-match factor. The modified error coefficients are used to correct the raw S-parameter data taken for the DUT.
In another aspect of the present invention, a method of compensating a calibration of a multiport vector network analyzer having switched receivers is provided. The method of compensating a calibration of a multiport network analyzer comprises characterizing a source match and a load match of each of a plurality of test ports of the VNA. The method of compensating further comprises computing a test port delta-match factor from each characterized source match and load match. The method further comprises computing a set of receiver switching factors for each test port. The method further comprises compensating error coefficients of a multi-term error model using the delta-match factor, and computing some of the error coefficients of the multi-term error model from other error coefficients using the receiver switching factors. In yet another aspect of the present invention, a vector network analyzer (VNA) comprising a compensated calibration according to the present invention is provided.
Advantageously, the compensation of the present invention is accomplished without resorting to the use of precision dual reflectometers at each VNA test port and without requiring a complete calibration of each pair of ports in a multiport configuration. In particular, it is unnecessary to characterize each possible combination of receiver and source port in a switched receiver, multiport VNA according to the present invention. Instead, the present invention utilizes a characterization of the source/load match differences for the VNA test ports that is independent of the calibration that is to be compensated. Thus, not only may any multi-term error model calibration be used with the invention, but also the present invention may be implemented as a xe2x80x98factory calibrationxe2x80x99 and/or a xe2x80x98field calibrationxe2x80x99. Furthermore, the present invention can be realized in existing VNAs as a firmware upgrade without requiring modification of the VNA hardware.
Moreover, the compensation of the present invention facilitates the use of eight-term calibration approaches in VNAs that exhibit differences in source and load matches at the test ports without the ability to monitor and/or remove those differences. Since monitoring the differences in source and load matches are often associated with higher cost VNAs, the present invention can provide a more economical solution to obtaining accurate S-parameter measurements for DUTs, especially so-called xe2x80x98non-connectorizedxe2x80x99 DUTs, where eight-term error model calibrations are preferred over the twelve-term calibration. Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings.